The present invention generally relates to a charge coupled device apparatus and, in particular, relates to one such apparatus having means for correcting for changes in the charge transfer efficiency of the charge coupled device (CCD).
Charge coupled devices are well known in the an and are widely used in the field of image processing. These devices are frequently used to provide electronic signal representations of incident electromagnetic radiation intensities of a particular spectral band. The most common spectral band used for CCD image processing is the visible band.
In general, the more conventional CCD's are fabricated from silicon in order to take advantage of the inherent characteristics of the silicon crystal lattice. Each atom of crystalline silicon is covalently bonded to its neighboring silicon atom resulting in an energy band gap of about 1.12 eV. To break one of the covalent bonds, and thereby create an electron-hole pair, an energy greater than the band gap energy is required. Electromagnetic radiation in the form of photons, having wavelengths in the visible spectrum possess such energy. Hence, light in the visible spectrum results in the creation of electron-hole pairs in the silicon lattice in relation to the intensity, i.e., the number of photons per unit of area, of the incident radiation.
In order to measure the electronic charge produced by the incident photons in such a CCD a means for collecting the charge is provided. Typically, this collecting means is provided in the form of a conductive gate structure formed in an array of rows and columns. This array of rows and columns defines an array of image elements generally referred to as pixels. Thus, it becomes possible, by applying the appropriate electrical potential to the various gate elements, to cause the free electrons created by incident photons to migrate along a row, or column. In this fashion an electronic signal representative of the intensity of the light radiation impinging upon the pixels can be provided as an output of the array. It is important to recognize that the electronic charges from the pixels furthest away from the output of the array undergo thousands of transfers depending on the size of the array.
Hence, it will be recognized that the charge transfer efficiency, i.e., the efficiency with which an electronic charge is moved from one pixel to an adjacent pixel is a critical parameter of a CCD. The charge transfer efficiency of a CCD becomes of particular importance when extremely low light levels are being sensed.
Although many environments can cause changes in the charge transfer efficiency of a CCD one of the more severe environments is outer space. For example, an outer space environment exposes a CCD to random high energy radiation and particle bombardment. Such conditions generally result in changes to the charge transfer efficiency of a CCD that are not only of unknown magnitude but are, consequently, uncompensated for when the image received by the CCD is processed.
In one particular application, CCDs are used to image stars for navigational purposes. For example, in order to accurately locate a space vehicle in outer space three points of reference are needed. Such points of reference are, most frequently, determined by the use of star trackers. A star tracker, in simplistic terms, is a device on a space vehicle that images stars and provides such images to an on-board processing unit that compares the position of the stars image provided by the CCD with pre-programmed celestial information. By use of information from the star tracker, preferably from a plurality of directions, the on-board processing unit calculates the precise location of the vehicle in space. Generally, if the location determined by the on-board processing unit does not match the anticipated, or predicted, location for the vehicle, the vehicle is commanded to move until the measured location is the same as the anticipated, or predicted, location.
An uncompensated change in the charge transfer efficiency of the CCD can result in images that misrepresent the position of a star or other celestial body. Under such conditions, the on-board processing unit would be provided with erroneous star location information. Thus, the position of the space vehicle would be erroneously determined and would not accurately match the anticipated, or predicted, location for the vehicle and, by compensating to match the measured location to the anticipated, or predicted, location, the vehicle could, in fact, be caused to move off course.
Consequently, it is highly desirable to provide a charge coupled device apparatus having means for correcting for changes in the charge transfer efficiency of the charge coupled device.